While not illegal, New Zealand had made it clear to Japan's Government that any such action would be unwelcome and offensive.

The Japanese Ambassador in Wellington received a dressing down yesterday, and today the Prime Minister John Key went further saying he would like an official apology.

"That would be good," he said.

"We'll see what happens from here, but whether there's an apology – we'll wait and see."

The NZ foreign ministry said the ship did not enter New Zealand's territorial waters, which extend 12 nautical miles from the coast, but did breach its EEZ, which covers from 12 to 200 nautical miles offshore.

NZ foreign minister Murray McCully earlier said New Zealand was strongly opposed to Japanese whaling in the southern oceans, and further action might be taken.

Japan's government spokesman defended the whaling ship's intrusion into the EEZ.

"We believe there was no problem with the movements of our ships from the point of view of international law," said Yoshihide Suga, Japan's chief cabinet secretary.

"This ship was taking protective moves and intruded on the EEZ as one part of that."

Japan, which says that whaling is a cherished cultural tradition, hunts whales under a scientific research provision in the moratorium on whaling.

New Zealand has supported Australia's case against Japanese whaling in the Southern Ocean in the International Court of Justice.